If you have memory set to "Auto" your mobo is choosing your memory devider for you and your memory could be running at any speed, really... even lower than 266, (although it will pick the devider that has it running closest to 266). Try playing around with that setting a bit yourself, although I doubt you'll be able to run a 1:1 ratio at 166 FSB with PC2100 without giving it a *lot* of voltage and relaxing your timings.